Great sleuthing, everyone!
I may be wrong, but I think this situation has arisen as part of the work we’re doing on improving the invite system. We are normalizing invites so they all use the same invitation system, and separating out the functions dealing with existing users. Most recently, we changed it so we notify existing users about topics now via the share button in the topic menu, separately from inviting new users to join the site and contribute to the topic.
We’ll want to do the same for groups - it’s already on our list to work on this, but now I guess it’s become more urgent if it means it’s not possible to add anyone to a group if you don’t have sufficient TL to invite new users to the site.